104 PROFESSOR PIAZZI SMYTH ON 
Hydrogen are the only ones he mentions seeing in the Hydrogen tube ; while amongst them the palm of 
brilliancy is is not with the red, as it is so often in small sparks, but in the more refrangible region of 
the glaucous Hydrogen line at 52, 255 W.N. Place. 
M. Frevez’ observations then were conducted on an electric stage quite above that on which I worked ; 
and he shows how any one else may attain tothe same. I will therefore only add, that I believe there is 
another stage below mine again, which would yield most important results for some of the physics of the 
faint Cometary, and Sidereal systems, could it be practically realised and well worked ; witness the fol- 
lowing very recent case for it. 
M. Jamin lately showed, in the Academy of Sciences in Paris, that the origination of the ‘‘proper” 
light in a Comet’s tail, must be the illumination of its carburetted constituent molecules by electric dis- 
charges of some kind, mainly because the only other known possible method of illumination, viz. by 
combustion, was absurd and utterly inapplicable under the circumstances, Professor Youne, of Prince- 
ton, U.S., the present Astronomer Royal at Greenwich, and others have on the contrary proved, by 
observation, and spectroscopic measurement of place, that the carburetted spectrum exhibited by a 
Comet’s tail is not that of the carbon-band order of electric illumined gas-vacuum tubes,—but 7s that of 
combustion of coal-gas and common air in the blue base of any ordinary burning flame. 
Now, as mentioned in the following pages, I have already found, on merely shortening and thicken- 
ing the wire forming the outer helix of my very moderate induction coil, (and thereby reducing the 
intensity of its sparks) that the brighter features of certain carbo-hydrogen combustion bands could be seen 
in an olefiant-gas vacuum tube, and less than before of the carbon band electric-lighted tube spectrum as 
usually known. Could we then,—by employing some very different method to the induction coil, of 
producing luminous electricity, as by the friction machine, Hotrz’s machine, or others,—so much further 
still reduce the intensity, while still keeping up the quantity, of the illuminating spark, as to render 
visible to us the combustion spectrum only, without any trace of the only hitherto known tube spectrum 
(which is the electric carbon-band spectrum) of a carbo-hydrogen gas—we should accomplish this ; viz., 
we should have reached the chief physical conditions of visibility of such a Comet as TEBBUT?’s great 
Comet of 1881, and harmonised at the same time the present apparently utter oppositions of M. Jamin’s 
theory versus Professor Younc’s and Mr Curistre’s observations. 
Or Pror. ALex. S. HEerscueu’s CoNTRIBUTION OF APPENDIX III. 
With the same spectroscope, tubes, and sparking apparatus employed by my- 
self, but with more powerful prisms inserted, and also some new tubes of his 
own, many observations have been made from time to time con amore by my 
friend Prof. Atex. S. HerscHEL; and when I found that he had very original 
theoretical ideas as to the arrangement of the lines and bands in many spectra, 
I invited him to lose no time in communicating them to the Royal Society, 
Edinburgh. This he kindly promised to do, if agreeable to the Society ; and 
although several other modes of presenting his views occurred to us, and were 
discussed, he preferred the method of contributing an Appendix to the present 
paper; on the clear understanding, however, that he is not necessarily bound 
by anything which I have written in the preceding part of this, or in any other, 
spectroscopic paper, but by his own portion only, viz., Appendix ITI. 
